Two-line hydraulic distribution systems have been widely used in aircraft as a means of minimizing hydraulic system weight. In such a system, one line transmits pressurized hydraulic fluid from the system pump to hydraulic actuators and/or motors in the aircraft which are employed to operate control surfaces, landing gear, etc. The remaining line returns the fluid from the actuators and/or motors to the system reservoir.
Typically, the hydraulic motors employed are axial piston motors, and even more typically, variable displacement, axial piston motors. Regardless, the hydraulic motors or actuators used in the system may be subject to cavitation when an aiding load comes into existence. In this situation, the aiding load tends to drive the hydraulic motor or actuator, thereby converting it into a pump. As a consequence, a low pressure will come into existence at the return port of the motor (which is the suction side of the motor when acting as a pump) or actuator which in turn can cause cavitation to occur. Further, where the motor or actuator is an axial piston motor, there is the additional danger of loss of the hydrostatic film on the wobbler as well as slipper hammering upon the wobbler and tipping of the slipper relative to the wobbler, all of which can accelerate wear.
More specifically, cavitation within the cylinder block bores and conventional kidney plate will occur when the motor return pressure drops to the partial pressure of the hydraulic fluid used in the system. As noted, the problem normally occurs when the motor is backdriven by an aiding load. In such a case, the motor control will force the motor to pump fluid from the return line at low pressure to the supply line at high pressure to absorb the energy provided by the aiding load and protect the motor from over-speeding.
The loss of hydraulic film between the slipper and the wobbler occurs if there is no pressure differential between the motor return pressure and the motor case drain pressure. Further, the lack of such a pressure differential will result in the slippers on the axial pistons tipping (due to centrifical force) and hammering as they return to contact with the wobbler as the pistons cross over from the low pressure or return side of the fluid distribution system to the high or supply pressure side of the motor. As noted, this causes premature wear, particularly of the slippers, and can result in the generation of debris as a result of such wear.
The present invention is directed to overcoming one or more of the above problems.